Overview
Rheumatoid arthritis (RA) is a complex disease of uncertain etiology which is distinguished by chronic inflammation of the joints. This disease ultimately leads to massive destruction of articular structures and is particularly important in the destruction of cartilage, the key load-bearing tissue of the joint (cf Isselbacher K. J. et al (1994) Harrison's Principles of Internal Medicine, McGraw-Hill, New York City (NYC) pp. 1648-55).
RA has been most intensively studied in the knee joint. Examination of the rheumatoid knee reveals a far different population of cells than is present in the normal knee. In RA, striking disruption of the regular organization of the normal synovial lining results in the invasion of tissue called pannus into the joint space. Pannus is comprised largely of fibroblast cells; however, the fibroblasts of the pannus exhibit an activated phenotype quite different from normal synovial fibroblasts. RA joints are also characterized by the abnormal occurrence of T and B lymphocytes, monocyte/macrophages, and mast cells. There is evidence that each of these cell types plays an important role in the chronic pathology of RA.
There is strong epidemiological evidence for T-cell involvement in RA. Significant correlations have been demonstrated between several HLA-DR genotypes and susceptibility to RA. These correlations have been taken as evidence of an immunological aspect to RA, and several models have been developed. These models have in common the generation of an immune response against antigens present in the rheumatoid joint. There is some evidence that the initial response may have been to viral antigens. In this scenario, ongoing immune response may be due to low levels of antigen persisting in the joint or a crossreaction to joint structures. Alternatively, the immunologic activity observed in RA may occur in response to the gradual damage and exposure of joint structures.
Evidence also points to the B lymphocyte as an important player in the etiology of RA. A hallmark of RA is the presence of rheumatoid factor (RF), an immune complex between disease-associated polyclonal antibodies and monospecific antibody which recognizes autologous Fc. These polyclonal antibodies may be induced by any number of sources with the subsequent production of antigen as discussed above.
The macrophage is almost certainly an important effector cell in the pathological mechanisms initiated by T and/or B lymphocytes. The macrophage elicits destructive mechanisms by elaborating powerful oxidants and a range of tissue destroying proteases. The macrophage also secretes a variety of cytokines which in turn recruit and activate other inflammatory cells.
Cellubrevin
Cellubrevins are homologues of synaptobrevins, synaptic vesicle-associated membrane proteins (VAMPs). Synaptobrevin was first discovered in rat brain (Baumert et al (1989) Embo J 8:379-84) and initially thought to be limited to neuronal cells. Synaptobrevin is an integral membrane protein of 18 kDA (Ralston E. et al (1994) J Biol Chem 269:15403-6) involved in the movement of vesicles from the plasmalemma of one cell, across the synapse, to the plasma membrane of the receptive neuron. This pathway and the endocytotic process may be blocked by the highly specific action of clostridial neurotoxins which prevents neurotransmitter release by cleaving the synaptobrevin molecule. Synaptobrevins are now known to occur and function in the receptor-mediated endocytotic pathways of many non-neuronal cell types.
Cellubrevins are 16 kDa proteins first found and investigated in rat cells and tissues (McMahon H. T. et al (1993) Nature 364:346-9). In vitro studies of various cellular membranes (Galli et al (1994) J Cell Biol 125:1015-24; Link et al (1993) J Biol Chem 268:18423-6) have shown that VAMPS including the cellubrevins are widely distributed and are important in membrane trafficking. They appear to participate in axon extension via exocytosis during development, in the release of neurotransmitters and modulatory peptides, and in endocytosis. Endocytotic vesicular transport includes such intracellular events as the fusions and fissions of the nuclear membrane, endoplasmic reticulum, Golgi apparatus, and various inclusion bodies (peroxisomes, lysosomes, etc).
Endocytotic processes appear to be universal in eukaryotic cells as diverse as yeast, Caenorhabditis elegans, Drosophila, and humans. The homologous proteins which direct the movement of vesicles within and between the cells of these organisms contain evolutionarily conserved domains. Generally, VAMPs have a three domain organization. The domains include a variable proline-rich, N-terminal sequence of 28 amino acids, a highly conserved central hydrophilic core of 69 amino acids, and a hydrophobic sequence of 23 amino acids presumed to be the membrane anchor.
As mentioned for synaptobrevin above, cellubrevins are sensitive to selective proteolysis by metalloendoproteases such as the zinc endoprotease which comprises the light chain of tetanus toxin. Experiments have shown that endosome fusion may continue even after specific cellubrevin cleavage through temperature- and ATP-dependent docking and fusion processes involving N-ethylmaleimide-sensitive fusion proteins (NSF) and small, soluble attachment proteins (SNAP).
A new cellubrevin homolog was discovered among the partial cDNAs which characterize rheumatoid synovium, Incyte Clone No. 80184. Because tissue distribution and VAMPs are more numerous and widely distributed than initially recognized, research on their differential expression and subcellular localization may turn out to be one of the most fruitful areas for the control or amelioration of diseases and disease symptoms.
Cellubrevins are associated with particular cell types, participate in both intracellular and extracellular pathways, and appear to vary in their abundance and specificity. Elucidation of the interactions of this novel homolog (and associated VAMPs) with docking proteins such as syntaxin and SNAPs of the plasmalemma or the core fusion proteins such as NSF and the synaptotagmins (Bark I. C. and Wilson M. C. (1994) Proc Natl Acad Sci 91:4621-4624) provides a means for the regulation of vesicle trafficking in normal as well as acute and chronic disease situations.